It started with a tipoff. I was reporting on the trafficking and exploitation of migrant workers in the Gulf when a source I had known for more than a decade reached out. They told me that child sexual abuse trafficking in the US was surging. As the Covid pandemic pushed predators online, some were using Facebook and Instagram to buy and sell children.

It was 2021 and I was about to begin an investigation with Mei-Ling McNamara, a human rights journalist, that would lead to the tech company Meta losing a multimillion-pound court case in March this year. The company had not yet rebranded and was known as Facebook, and there had not been any reporting on how children were being trafficked on its platforms. Experts from anti-trafficking nonprofit organisations and an American law enforcement official talked me through the crimes they were seeing.

Much of the trafficking on Facebook and Instagram was happening in non-public areas of the platforms, such as Facebook Messenger and private Instagram accounts, I would learn later. Traffickers were searching for teens to target and groom, and to later advertise to sex buyers.

Sex trafficking is the use of force, fraud or coercion in the buying and selling of non-consensual sex acts, whether or not travel is involved. Under international law, children cannot legally consent to any kind of sex act, therefore anyone who profits from or pays for a sex act from a child – including profiting from or paying for photographs depicting sexual exploitation – is considered a human trafficker.

One of the best investigative tools for obtaining documents on trafficking cases is Pacer, the federal courts records database. However, finding evidence is not straightforward. Pacer does not have a text search function and many cases involving child exploitation have sealed records. Instead, I had to search the Department of Justice press releases for trafficking cases that might involve social media. I spent hours trawling through criminal complaints, transcripts and exhibit filings for these cases on Pacer. The results were often shocking.

I was able to pull transcripts of sale negotiations for teen girls that traffickers were engaging in on Facebook Messenger, the private messaging function. In exhibit documents, there were pictures of trafficking victims being advertised for sale in Instagram’s Stories function. Money and logistics had been discussed. In the cases we found, none of these crimes had been detected or flagged by Meta.

McNamara and I contacted former contract workers who had been employed to moderate Facebook and Instagram, tasked with reporting and removing harmful content. Many were traumatised by the content they had had to review each day. All said their efforts to flag and escalate possible child trafficking on Meta platforms often went nowhere, and harmful content was rarely taken down by the company. They felt helpless, and believed Meta’s criteria for escalating possible crimes to law enforcement was too narrow.

In July 2022, we went to Washington DC to visit a safe house run by the nonprofit Courtney’s House, which cares for teen girls of colour who are survivors of trafficking or are actively being trafficked.

Its location is not public and we were only sent the address an hour before our appointment. Courtney’s House is run by Tina Frundt, a trafficking survivor and former member of the United States Advisory Council on Human Trafficking during the Obama administration.

We sat down on the sofas in the living room and recorded our hours-long discussion about how teen girls are targeted by sex traffickers. Frundt showed us how Instagram’s Stories function was used by traffickers to advertise girls for sex. She spoke in detail about how girls and LGBTQ+ youth were targeted, how a family member was involved or complicit in their trafficking, in some cases. Then she fell silent for a moment and drew a breath.

There was a 15-year-old girl who used to come to Courtney’s House. She was popular with the other girls, she loved to dance, play board games and swap makeup tips with Frundt. She was broken by what she had been through, but was deeply loved by her family and the others at Courtney’s House, Frundt said. Then in June 2021 she met a sex buyer who had connected with her on Instagram. This 43-year-old man had given her fentanyl-laced drugs. She went to bed that night and never woke up. We gave her the alias Maya in the investigation to protect the privacy of her family.

On another reporting trip, we visited an assistant district attorney’s office in Massachusetts. As we spoke about the issues they were seeing – that child trafficking crimes on social media platforms were increasing at a rate of about 30% each year – two police officers and a cyber intelligence analyst also joined us. The pandemic only made things worse, as children were learning from home, spent more time online, and were not in direct contact with teachers and other adults who might have noticed if something was amiss.

For traffickers, it was easy to spot the most vulnerable children who would be easiest to target, groom and exploit based on their activity online, the prosecutor said.

“We’re seeing more and more people with significant criminal records move into this area. It’s incredibly lucrative,” said the prosecutor. “Now, all the appointments are set up online. The money can be exchanged digitally. Everything is done seamlessly by the traffickers.”

We talked about some of their investigations, and ways in which Meta had been used by traffickers to identify potential victims and connect with them. We interviewed more prosecutors. An incarcerated sex trafficker told us about how Instagram was his platform of choice to commit his crimes.

From the reporting, it became clear to us that Meta was struggling to prevent criminals using its platforms to buy and sell children for sex. The company vigorously disputed the allegations brought forth by our investigation.

The investigation was published in April 2023, titled How Facebook and Instagram became marketplaces for child sex trafficking. Initially, it was not clear if the piece had made much impact. In the US, social media platforms are shielded from legal liability for crimes committed using their platforms by a federal law called Section 230, as long as they are unaware of that content’s existence.

However, several months later, we learned that the investigation had been cited in a supreme court amicus brief. At the same time, New Mexico’s office of the attorney general filed a lawsuit against the company for failing to protect children from sexual abuse and human trafficking on its platforms.

The complaint stated: “Meta has allowed Facebook and Instagram to become a marketplace for predators in search of children upon whom to prey.” Our investigation was cited several times in the court document.

The case went to trial this year: the first jury trial Meta has faced. The company lost the court battle in March and was ordered to pay $375m (£281m) in civil penalties for violating New Mexico’s consumer protection laws. Meta said it would appeal against the ruling and that it remained “confident in our record of protecting teens online”.

In the three years since the first investigation was published, the Guardian has continued to publish fresh revelations of how children and teens have been exploited and trafficked over Meta’s platforms.

They include that Facebook’s private messaging platform Messenger and its payment platform Meta Pay were being used by traffickers to exchange money for child sexual abuse material. Several articles were published about Kristen Galvan, a teenage girl from Texas who was groomed and sold for sex by her traffickers using Instagram. She had been missing since 2020. This year, the Guardian published an article revealing she had been murdered, and that her partial remains had been located. Her killers have never been caught.

Child safety experts and law enforcement have long criticised Meta’s December 2023 move to encrypt Facebook Messenger, to enhance privacy for its users. Encryption ensures that only the sender and intended recipient can view messages by converting them into unreadable code that is decrypted upon receipt. The messages cannot be scanned for inappropriate content, or viewed by the company or law enforcement.

Meta has previously defended encryption as safe because users can report any inappropriate interactions or abuse they experience while using Messenger.

Yet, when Adam Mosseri, the head of Instagram, took the stand, he stated that self-reporting tools were far less effective than the company’s own detection technology, directly contradicting Meta’s official stance. He also discussed previously abandoned plans to encrypt Instagram’s direct messages, noting that doing so would have made it harder to protect children on the platform.

Meta’s difficulties with detecting and reporting child exploitation on its platforms were discussed in detail in the trial. The Guardian reported that law enforcement had been flooded with “junk” tips from the company, which hindered investigations.

Just one day after the verdict in New Mexico, Meta lost another trial in Los Angeles, where it came under fire for platform features that impact children’s mental health by being intentionally addictive and amplifying content promoting self-harm, suicidal ideation and body dysmorphia. Meta has said it will appeal against the ruling, saying “we will continue to defend ourselves vigorously as every case is different, and we remain confident in our record of protecting teens online.”

More trials are likely to come. Meta’s next court battle will probably be against a coalition of 33 attorneys general, alleging the company “knowingly designed and deployed harmful features” that “purposefully addict children and teens”.